Inhibitory effects of activin-a on osteoblast differentiation during cultures of fetal rat calvarial cells

HSPB7 oppositely regulates human mesenchymal stromal cell-derived osteogenesis and adipogenesis

BACKGROUND

Recent evidence suggests that accumulation of marrow adipose tissue induced by aberrant lineage allocation of bone marrow-derived mesenchymal stromal cells (BMSCs) contributes to the pathophysiologic processes of osteoporosis. Although master regulators of lineage commitment have been well documented, molecular switches between osteogenesis and adipogenesis are largely unknown.

METHODS

HSPB7 gene expression during osteogenic and adipogenic differentiation of BMSCs was evaluated by qPCR and Western blot analyses. Lentiviral-mediated knockdown or overexpression of HSPB7 and its deletion constructs were used to assess its function. The organization of cytoskeleton was examined by immunofluorescent staining. ALP activity, calcium assay, Alizarin Red S staining and Oil Red O staining were performed in vitro during osteoblast or adipocyte differentiation. SB431542 and Activin A antibody were used to identify the mechanism of Activin A in the regulation of osteogenic differentiation in BMSCs.

RESULTS

In this study, we identified HSPB7 capable of oppositely regulating osteogenic and adipogenic differentiation of BMSCs. HSPB7 silencing promoted adipogenesis while reducing osteogenic differentiation and mineralization. Conversely, overexpression of HSPB7 strongly enhanced osteogenesis, but no effect was observed on adipogenic differentiation. Deletion of the N-terminal or C-terminal domain of HSPB7 led to decreased osteoblastic potency and mineralization. Mechanistically, our data showed that Activin A is a downstream target participating in HSPB7 knockdown-mediated osteogenic inhibition.

CONCLUSIONS

Our findings suggest that HSPB7 plays a positive role in driving osteoblastic differentiation, and with the capability in maintaining the osteo-adipogenesis balance. It holds great promise as a potential therapeutic target in the treatment of bone metabolic diseases.

Granular PEG hydrogels mediate osteoporotic MSC clustering via N-cadherin influencing the pro-resorptive bias of their secretory profile

Postmenopausal osteoporosis results from a pro-resorptive bone environment, which decreases bone mineral density causing increased fracture risk. Bone marrow derived mesenchymal stem/stromal cells (MSCs) secrete factors involved in bone homeostasis, but osteoporosis mediated changes to their secretions remain understudied. Herein, we examined the secretome of MSCs isolated from ovariectomized rats (OVX rMSCs), a model of post-menopausal osteoporosis, as a function of cell-cell interactions. Specifically, we controlled clustering of OVX and SHAM rMSCs by assembling them in granular hydrogels synthesized from poly(ethylene glycol) microgels with average diameters of ∼10, 100, and 200 µm. We directed both the sizes of rMSC clusters (single cells to ∼30 cells/cluster) and the percentages of cells within clusters (∼20-90%) by controlling the scaffold pore dimensions. Large clusters of OVX rMSCs had a pro-resorptive secretory profile, with increased concentrations of Activin A, CXCL1, CX3CL1, MCP-1, TIMP-1, and TNF-ɑ, compared to SHAM rMSCs. As this pro-resorptive bias was only observed in large cell clusters, we characterized the expression of several cadherins, mediators of cell-cell contacts. N-cadherin expression was elevated (∼4-fold) in OVX relative to SHAM rMSCs, in both cell clusters and single cells. Finally, TIMP-1 and MCP-1 secretion was only decreased in large cell clusters of OVX rMSCs when N-cadherin interactions were blocked, highlighting the dependence of OVX rMSC secretion of pro-resorptive cytokines on N-cadherin mediated cell-cell contacts. Further elucidation of the N-cadherin mediated osteoporotic MSC secretome may have implications for developing therapies for postmenopausal osteoporosis. STATEMENT OF SIGNIFICANCE: Postmenopausal osteoporosis is a prevalent bone disorder that affects tens of millions of women worldwide. This disease is characterized by severe bone loss resulting from a pro-resorptive bone marrow environment, where the rates of bone resorption outpace the rates of bone deposition. The paracrine factors secreted by bone marrow MSCs can influence cell types responsible for bone homeostasis, but the osteoporosis-mediated changes to MSC secretory properties remains understudied. In this study, we used PEG-based porous granular scaffolds to study the influence of cell clustering on the secretory properties of osteoporotic MSCs. We observed increased secretion of several pro-resorptive factors by osteoporotic MSCs in large clusters. Further, we explored the dependence of this altered secretion profile on N-cadherin mediated cell-cell contacts.

Deletion of activin A in mesenchymal but not myeloid cells ameliorates disease severity in experimental arthritis

Objective

The aim of this study was to assess the extent and the mechanism by which activin A contributes to progressive joint destruction in experimental arthritis and which activin A-expressing cell type is important for disease progression.

Methods

Levels of activin A in synovial tissues were evaluated by immunohistochemistry, cell-specific expression and secretion by PCR and ELISA, respectively. Osteoclast (OC) formation was assessed by tartrat-resistant acid phosphatase (TRAP) staining and activity by resorption assay. Quantitative assessment of joint inflammation and bone destruction was performed by histological and micro-CT analysis. Immunoblotting was applied for evaluation of signalling pathways.

Results

In this study, we demonstrate that fibroblast-like synoviocytes (FLS) are the main producers of activin A in arthritic joints. Most significantly, we show for the first time that deficiency of activin A in arthritic FLS (ActβA^d/d ColVI-Cre) but not in myeloid cells (ActβA^d/d LysM-Cre) reduces OC development in vitro, indicating that activin A promotes osteoclastogenesis in a paracrine manner. Mechanistically, activin A enhanced OC formation and activity by promoting the interaction of activated Smad2 with NFATc1, the key transcription factor of osteoclastogenesis. Consistently, ActβA^d/d LysM-Cre hTNFtg mice did not show reduced disease severity, whereas deficiency of activin A in ColVI-Cre-expressing cells such as FLS highly diminished joint destruction reflected by less inflammation and less bone destruction.

Conclusions

The results highly suggest that FLS-derived activin A plays a crucial paracrine role in inflammatory joint destruction and may be a promising target for treating inflammatory disorders associated with OC formation and bone destruction like rheumatoid arthritis.

Treatment with Soluble Activin Type IIB Receptor Ameliorates Ovariectomy-Induced Bone Loss and Fat Gain in Mice

INTRODUCTION

In postmenopausal osteoporosis, hormonal changes lead to increased bone turnover and metabolic alterations including increased fat mass and insulin resistance. Activin type IIB receptors bind several growth factors of the TGF-β superfamily and have been demonstrated to increase muscle and bone mass. We hypothesized that ActRIIB-Fc treatment could improve bone and muscle mass, inhibit fat accumulation, and restore metabolic alterations in an ovariectomy (OVX) model of postmenopausal osteoporosis.

MATERIALS AND METHODS

Female C57Bl/6 N mice were subjected to SHAM or OVX procedures and received intraperitoneal injections of either PBS or ActRIIB-Fc (5 mg/kg) once weekly for 7 weeks. Glucose and insulin tolerance tests (GTT and ITT, respectively) were performed at 7 and 8 weeks, respectively. Bone samples were analyzed with micro-computed tomography imaging, histomorphometry, and quantitative RT-PCR.

RESULTS

Bone mass decreased in OVX PBS mice compared to the SHAM PBS group but ActRIIB-Fc was able to prevent these changes as shown by µCT and histological analyses. This was due to decreased osteoclast numbers and function demonstrated by histomorphometric and qRT-PCR analyses. OVX induced adipocyte hypertrophy that was rescued by ActRIIB-Fc, which also decreased systemic adipose tissue accumulation. OVX itself did not affect glucose levels in GTT but ActRIIB-Fc treatment resulted in impaired glucose clearance in both SHAM and OVX groups. OVX induced mild insulin resistance in ITT but ActRIIB-Fc treatment did not affect this.

CONCLUSION

Our results reinforce the potency of ActRIIB-Fc as a bone-enhancing agent but also bring new insight into the metabolic effects of ActRIIB-Fc in normal and OVX mice.

Activin A Promotes Osteoblastic Differentiation of Human Preosteoblasts through the ALK1-Smad1/5/9 Pathway

Activin A, a member of transforming growth factor-β superfamily, is involved in the regulation of cellular differentiation and promotes tissue healing. Previously, we reported that expression of activin A was upregulated around the damaged periodontal tissue including periodontal ligament (PDL) tissue and alveolar bone, and activin A promoted PDL-related gene expression of human PDL cells (HPDLCs). However, little is known about the biological function of activin A in alveolar bone. Thus, this study analyzed activin A-induced biological functions in preosteoblasts (Saos2 cells). Activin A promoted osteoblastic differentiation of Saos2 cells. Activin receptor-like kinase (ALK) 1, an activin type I receptor, was more strongly expressed in Saos2 cells than in HPDLCs, and knockdown of ALK1 inhibited activin A-induced osteoblastic differentiation of Saos2 cells. Expression of ALK1 was upregulated in alveolar bone around damaged periodontal tissue when compared with a nondamaged site. Furthermore, activin A promoted phosphorylation of Smad1/5/9 during osteoblastic differentiation of Saos2 cells and knockdown of ALK1 inhibited activin A-induced phosphorylation of Smad1/5/9 in Saos2 cells. Collectively, these findings suggest that activin A promotes osteoblastic differentiation of preosteoblasts through the ALK1-Smad1/5/9 pathway and could be used as a therapeutic product for the healing of alveolar bone as well as PDL tissue.

Principles of the activin receptor signaling pathway and its inhibition

This review captures the anabolic and stimulatory effects observed with inhibition of the transforming growth factor β superfamily in muscle, blood, and bone. New medicinal substances that rectify activin, myostatin, and growth differentiation factor 11 signaling give hope to the many whose lives are affected by deterioration of these tissues. The review first covers the origin, structure, and common pathway of activins, myostatin, and growth differentiation factor 11 along with the pharmacodynamics of the new class of molecules designed to oppose the activin receptor signaling pathway. Current terminology surrounding this new class of molecules is inconsistent and does not infer functionality. Adopting inhibitors of the activin receptor signaling pathway (IASPs) as a generic term is proposed because it encapsulates the molecular mechanisms along the pathway trajectory. To conclude, a pragmatic classification of IASPs is presented that integrates functionality and side effects based on the data available from animals and humans. This provides researchers and clinicians with a tool to tailor IASPs therapy according to the need of projects or patients and with respect to side effects.

Novel ActRIIB ligand trap increases muscle mass and improves bone geometry in a mouse model of severe osteogenesis imperfecta

Osteogenesis imperfecta (OI) caused by mutations affecting the extracellular matrix protein collagen type I is characterized by fragile bones and low muscle mass and function. Activin A and myostatin, members of the TGF-β superfamily, play a key role in the control of muscle mass and in muscle-bone communication. Here we investigated activin A/myostatin signaling in a mouse model of severe dominant OI, Col1a1^Jrt/+mouse, and the effect of activin A/myostatin inhibition by a soluble activin receptor IIB receptor, ACE-2494, on bones and muscles in 8-week old mice. Compared to wild type mice, Col1a1^Jrt/+mice had elevated TGF-β signaling in bone and muscle tissue. ACE-2494 treatment of wild type mice resulted in significantly increased muscle mass, bone length, bone mass as well as improved bone mechanical properties. However, treatment of Col1a1^Jrt/+mice with ACE-2494 was associated with significant gain in muscle mass, significantly improved bone length and bone geometry, but no significant treatment effect was found on bone mass or bone mechanical properties. Thus, our data indicate that activin A/myostatin neutralizing antibody ACE-2494 is effective in stimulating muscle mass, bone length and diaphyseal bone growth but does not correct bone mass phenotype in a mouse model ofdominant OI.

Two-day-treatment of Activin-A leads to transient change in SV-HFO osteoblast gene expression and reduction in matrix mineralization

Activins regulate bone formation by controlling osteoclasts and osteoblasts. We investigated Activin‐A mechanism of action on human osteoblast mineralization, RNA and microRNA (miRNA) expression profile. A single 2‐day treatment of Activin‐A at Day 5 of osteoblast differentiation significantly reduced matrix mineralization. Activin A‐treated osteoblasts responded with transient change in gene expression, in a 2‐wave‐fashion. The 38 genes differentially regulated during the first wave (within 8 hr after Activin A start) were involved in transcription regulation. In the second wave (1–2 days after Activin A start), 65 genes were differentially regulated and related to extracellular matrix. Differentially expressed genes in both waves were associated to transforming growth factor beta signaling. We identified which microRNAs modulating osteoblast differentiation were regulated by Activin‐A. In summary, 2‐day treatment with Activin‐A in premineralization period of osteoblast cultures influenced miRNAs, gene transcription, and reduced matrix mineralization. Modulation of Activin A signaling might be useful to control bone quality for therapeutic purposes.

Regulation of Histone Deacetylases by MicroRNAs in Bone

Formation of new bone by osteoblasts is mediated via the activation of signaling pathways, such as TGF-β, BMP, and Wnt. A number of transcription factors participate in the signaling cascades that are tightly regulated by other regulatory factors. Histone deacetylases (HDACs) are one such class of regulatory factors that play an essential role in influencing chromatin architecture and regulate the expression of the genes that play a role in osteoblast differentiation by the mechanism of deacetylation. Four classes of HDACs have been identified namely, class I, class II A, class II B, class III and class IV. MicroRNAs (miRNAs) are small fragments of non-coding RNAs typically 19-25 nucleotides long that target mRNAs to upregulate or downregulate gene expression at a post-transcriptional level. A number of miRNAs that target HDACs in bone have been recently reported. Hence, in this review, we elaborate on the various miRNAs that target the different classes of HDACs and impact of the same on osteogenesis.

Activin-A is elevated in patients with thalassemia major and double heterozygous sickle cell/beta-thalassemia and correlates with markers of hemolysis and bone mineral density

Despite the advances in the management of hemoglobinopathies, further insight into disease pathophysiology is necessary to improve our therapeutic approach. Activin-A has emerged as a regulator of erythropoiesis and bone turnover in malignant disorders; however, clinical data in hemoglobinopathies are currently scarce. Thus, we aimed to investigate the role of activin-A among hemoglobinopathy patients and evaluate the rationale of its targeting. Circulating levels of activin-A were measured in patients (n = 227) with beta-thalassemia major (TM) (n = 58), beta-thalassemia intermedia (TI) (n = 43), double heterozygous sickle cell/beta-thalassemia (HbS/beta-thal) (n = 109), or homozygous sickle cell disease (n = 17), and we explored possible correlations with clinical and laboratory data. Seventeen age- and gender-matched, healthy individuals served as controls. Bone marrow density (BMD) was determined using dual-energy X-ray absorptiometry. TM and HbS/beta-thal patients had elevated activin-A compared to controls (p = 0.041 and p = 0.038, respectively). In TM patients, high circulating activin-A showed strong correlations with hemolysis markers, namely reticulocyte count (p = 0.011) and high lactate dehydrogenase (LDH; p = 0.024). Similarly, in HbS/beta-thal patients, activin-A showed positive correlations with indirect bilirubin (p < 0.001), ferritin (p = 0.005), and LDH (p = 0.044). High activin-A correlated with low Z-score of both lumbar spine BMD in TI patients (p < 0.01) and femoral neck BMD in TM patients (p < 0.01). Serum activin-A is elevated in patients with TM and HbS/beta-thal and correlates with markers of hemolysis and low BMD. These data support a role of activin-A in the biology of these disorders and provide further rationale for the broader clinical development of activin-A inhibitors in this setting.

A follistatin-based molecule increases muscle and bone mass without affecting the red blood cell count in mice

Inhibitors of the activin receptor signaling pathway (IASPs) have become candidate therapeutics for sarcopenia and bone remodeling disorders because of their ability to increase muscle and bone mass. However, IASPs utilizing activin type IIA and IIB receptors are also potent stimulators of erythropoiesis, a feature that may restrict their usage to anemic patients because of increased risk of venous thromboembolism. Based on the endogenous TGF-β superfamily antagonist follistatin (FST), a molecule in the IASP class, FST_ΔHBS-mFc, was generated and tested in both ovariectomized and naive BALB/c and C57BL/6 mice. In ovariectomized mice, FST_ΔHBS-mFc therapy dose-dependently increased cancellous bone mass up to 42% and improved bone microstructural indices. For the highest dosage of FST_ΔHBS-mFc (30 mg/kg, 2 times/wk), the increase in cancellous bone mass was similar to that observed with parathyroid hormone therapy (1-34, 80 µg/kg, 5 times/wk). Musculus quadriceps femoris mass dose-dependently increased up to 21% in ovariectomized mice. In both ovariectomized and naive mice, FST_ΔHBS-mFc therapy did not influence red blood cell count or hematocrit or hemoglobin levels. If the results are reproduced, a human FST_ΔHBS-mFc version could be applicable in patients with musculoskeletal conditions irrespective of hematocrit status.-Lodberg, A., van der Eerden, B. C. J., Boers-Sijmons, B., Thomsen, J. S., Brüel, A., van Leeuwen, J. P. T. M., Eijken, M. A follistatin-based molecule increases muscle and bone mass without affecting the red blood cell count in mice.

The BMP Pathway and Its Inhibitors in the Skeleton

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-β family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

VWC2 Increases Bone Formation Through Inhibiting Activin Signaling

By a bioinformatics approach, we have identified a novel cysteine knot protein member, VWC2 (von Willebrand factor C domain containing 2) previously known as Brorin. Since Brorin has been proposed to function as a bone morphogenetic protein (BMP) antagonist, we investigated the binding of Brorin/VWC2 to several BMPs; however, none of the BMPs tested were bound to VWC2. Instead, the βA subunit of activin was found as a binding partner among transforming growth factor (TGF)-β superfamily members. Here, we show that Vwc2 gene expression is temporally upregulated early in osteoblast differentiation, VWC2 protein is present in bone matrix, and localized at osteoblasts/osteocytes. Activin A-induced Smad2 phosphorylation was inhibited in the presence of exogenous VWC2 in MC3T3-E1 osteoblast cell line and primary osteoblasts. The effect of VWC2 on ex vivo cranial bone organ cultures treated with activin A was investigated, and bone morphometric parameters decreased by activin A were restored with VWC2. When we further investigated the biological mechanism how VWC2 inhibited the effects of activin A on bone formation, we found that the effects of activin A on osteoblast cell growth, differentiation, and mineralization were reversed by VWC2. Taken together, a novel secretory protein, VWC2 promotes bone formation by inhibiting Activin-Smad2 signaling pathway.

Systemic Activation of Activin A Signaling Causes Chronic Kidney Disease-Mineral Bone Disorder

The high cardiovascular mortality associated with chronic kidney disease (CKD) is caused in part by the CKD-mineral bone disorder (CKD-MBD) syndrome. The CKD-MBD consists of skeletal, vascular and cardiac pathology caused by metabolic derangements produced by kidney disease. The prevalence of osteopenia/osteoporosis resulting from the skeletal component of the CKD-MBD, renal osteodystrophy (ROD), in patients with CKD exceeds that of the general population and is a major public health concern. That CKD is associated with compromised bone health is widely accepted, yet the mechanisms underlying impaired bone metabolism in CKD are not fully understood. Therefore, clarification of the molecular mechanisms by which CKD produces ROD is of crucial significance. We have shown that activin A, a member of the transforming growth factor (TGF)-β super family, is an important positive regulator of receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis with Smad-mediated signaling being crucial for inducing osteoclast development and function. Recently, we have demonstrated systemic activation of activin receptors and activin A levels in CKD mouse models, such as diabetic CKD and Alport (AL) syndrome. In these CKD mouse models, bone remodeling caused by increased osteoclast numbers and activated osteoclastic bone resorption was observed and treatment with an activin receptor ligand trap repaired CKD-induced-osteoclastic bone resorption and stimulated individual osteoblastic bone formation, irrespective of parathyroid hormone (PTH) elevation. These findings have opened a new field for exploring mechanisms of activin A-enhanced osteoclast formation and function in CKD. Activin A appears to be a strong candidate for CKD-induced high-turnover ROD. Therefore, the treatment with the decoy receptor for activin A might be a good candidate for treatment for CKD-induced osteopenia or osteoporosis, indicating that the new findings from in these studies will lead to the identification of novel therapeutic targets for CKD-related and osteopenia and osteoporosis in general. In this review, we describe the impact of CKD-induced Smad signaling in osteoclasts, osteoblasts and vascular cells in CKD.

Mechanisms involved in enhancement of osteoclast formation by activin-A

Several growth factors in bone tissues are reported to be associated with osteoclastogenesis. Activin-A, a member of the transforming growth factor-β (TGF-β) family is known to be present in bone tissues and an important regulator in osteoclastogenesis with SMAD-mediated signaling being crucial for inducing osteoclast differentiation. In the present study, we examined the effect and underlying mechanisms of activin-A on osteoclast formation in vitro culture systems. Activin-A enhanced osteoclast formation in both mouse bone marrow cells and monocyte/macrophage cell line RAW 264.7 cells induced by receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL) and/or macrophage stimulating factor (M-CSF). We also found that activin-A stimulated bone resorption and actin ring formation induced by RANKL and/or M-CSF. Furthermore, activin-A enhanced RANKL-induced expression of nuclear factor of activated T cell cytoplasmic 1 (NFATc1), a key regulator of osteoclastogenesis, thereby increasing osteoclastogenesis-related marker gene expression, including tartrate-resistant acid phosphatase, osteoclast stimulatory transmembrane protein, and cathepsin K. Blockage of receptor binding by follistatin, an activing-binding protein suppressed the activin-A-mediated stimulation of NFATc1. In addition, activin-A increased RANKL-induced c-fos expression without significantly affecting the NF-κB and mitogen-activated protein kinase (MAPK) signaling pathway. Pre-treatment of the cells with a specific inhibitor of SMAD2/3 attenuated the activin-A-induced expression of NFATc1 and co-immunoprecipitation assay revealed that treatment with activin-A increased physical interaction of phosphorylated-c-fos and phosphorylated-SMAD2 protein induced by RANKL. These results suggest that activin-A enhances RANKL-induced osteoclast formation mediated by interaction of c-fos and smad2/3.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

A soluble activin type IIA receptor mitigates the loss of femoral neck bone strength and cancellous bone mass in a mouse model of disuse osteopenia

Disuse causes a rapid and substantial bone loss distinct in its pathophysiology from the bone loss associated with cancers, age, and menopause. While inhibitors of the activin-receptor signaling pathway (IASPs) have been shown to prevent ovariectomy- and cancer-induced bone loss, their application in a model of disuse osteopenia remains to be tested. Here, we show that a soluble activin type IIA receptor (ActRIIA-mFc) increases diaphyseal bone strength and cancellous bone mass, and mitigates the loss of femoral neck bone strength in the Botulinum Toxin A (BTX)-model of disuse osteopenia in female C57BL/6J mice. We show that ActRIIA-mFc treatment preferentially stimulates a dual-effect (anabolic-antiresorptive) on the periosteal envelope of diaphyseal bone, demonstrating in detail the effects of ActRIIA-mFc on cortical bone. These observations constitute a previously undescribed feature of IASPs that mediates at least part of their ability to mitigate detrimental effects of unloading on bone tissue. The study findings support the application of IASPs as a strategy to combat bone loss during disuse.

Clinical Relevance and Mechanisms of Antagonism Between the BMP and Activin/TGF-β Signaling Pathways

The transforming growth factor β (TGF-β) superfamily is a large group of signaling molecules that participate in embryogenesis, organogenesis, and tissue homeostasis. These molecules are present in all animal genomes. Dysfunction in the regulation or activity of this superfamily's components underlies numerous human diseases and developmental defects. There are 2 distinct arms downstream of the TGF-β superfamily ligands-the bone morphogenetic protein (BMP) and activin/TGF-β signaling pathways-and these 2 responses can oppose one another's effects, most notably in disease states. However, studies have commonly focused on a single arm of the TGF-β superfamily, and the antagonism between these pathways is unknown in most physiologic and pathologic contexts. In this review, the authors summarize the clinically relevant scenarios in which the BMP and activin/TGF-β pathways reportedly oppose one another and identify several molecular mechanisms proposed to mediate this interaction. Particular attention is paid to experimental findings that may be informative to human pathology to highlight potential therapeutic approaches for future investigation.

Role of follistatin in muscle and bone alterations induced by gravity change in mice

Interactions between muscle and bone have been recently noted. We reported that the vestibular system plays crucial roles in the changes in muscle and bone induced by hypergravity in mice. However, the details of the mechanisms by which gravity change affects muscle and bone through the vestibular system still remain unknown. Here, we investigated the roles of humoral factors linking muscle to bone and myostatin-related factors in the hypergravity-induced changes in muscle and bone in mice with vestibular lesions (VL). Hypergravity elevated serum and mRNA levels of follistatin, an endogenous inhibitor of myostatin, in the soleus muscle of mice. VL blunted the hypergravity-enhanced levels of follistatin in the soleus muscle of mice. Simulated microgravity decreased follistatin mRNA level in mouse myoblastic C2C12 cells. Follistatin elevated the mRNA levels of myogenic genes as well as the phosphorylation of Akt and p70S6 kinase in C2C12 cells. As for bone metabolism, follistatin antagonized the mRNA levels of osteogenic genes suppressed by activin A during the differentiation of mesenchymal cells into osteoblastic cells. Moreover, follistatin attenuated osteoclast formation enhanced by myostatin in the presence of receptor activator of nuclear factor-κB ligand in RAW 264.7 cells. Serum follistatin levels were positively related to bone mass in mouse tibia. In conclusion, the present study provides novel evidence that hypergravity affects follistatin levels in muscle through the vestibular system in mice. Follistatin may play some roles in the interactions between muscle and bone metabolism in response to gravity change.

Role of Osteogenic Growth Peptide (OGP) and OGP(10-14) in Bone Regeneration: A Review

Bone regeneration is a process that involves several molecular mediators, such as growth factors, which directly affect the proliferation, migration and differentiation of bone-related cells. The osteogenic growth peptide (OGP) and its C-terminal pentapeptide OGP(10–14) have been shown to stimulate the proliferation, differentiation, alkaline phosphatase activity and matrix mineralization of osteoblastic lineage cells. However, the exact molecular mechanisms that promote osteoblastic proliferation and differentiation are not completely understood. This review presents the main chemical characteristics of OGP and/or OGP(10–14), and also discusses the potential molecular pathways induced by these growth factors to promote proliferation and differentiation of osteoblasts. Furthermore, since these peptides have been extensively investigated for bone tissue engineering, the clinical applications of these peptides for bone regeneration are discussed.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

Recent proteomic advances in developmental, regeneration, and cancer governing signaling pathways

Embryonic development, adult tissue repair, and cancer share a number of common regulating pathways. The basic processes that govern the events that induce mesenchymal properties in epithelial cells-a process known as epithelial-mesenchymal transition-are central for embryonic development, and can be resumed in adults either during wound healing or tissue regeneration. A misregulation of these pathways is involved in pathological situations, such as tissue fibrosis and cancer. Proteomic approaches have emerged as promising tools to better understand the signaling pathways that govern these complex biological processes. This review focuses on the recent proteomic-based contributions to better understand the modulation of transforming growth factor-beta (TGF-β), wingless-type MMTV integration site family (Wnt), Notch and Receptor tyrosine kinase (RTK) signaling pathways. New advances include the description of new protein interactions, the formation of new protein complexes or the description on how some PTMs are regulating these pathways. Understanding protein interactions and the tempo-spatial modulation of these pathways might lead us to interesting research quests in cancer, embryonic development or even on improving adult tissue regeneration capabilities.

Engineering TGF-β superfamily ligands for clinical applications

TGF-β superfamily ligands govern normal tissue development and homeostasis, and their dysfunction is a hallmark of many diseases. These ligands are also well defined both structurally and functionally. This review focuses on TGF-β superfamily ligand engineering for therapeutic purposes, in particular for regenerative medicine and musculoskeletal disorders. We describe the key discovery that structure-guided mutation of receptor-binding epitopes, especially swapping of these epitopes between ligands, results in new ligands with unique functional properties that can be harnessed clinically. Given the promising results with prototypical engineered TGF-β superfamily ligands, and the vast number of such molecules that remain to be produced and tested, this strategy is likely to hold great promise for the development of new biologics.

Effects of Activin A on the phenotypic properties of human periodontal ligament cells

Periodontal ligament (PDL) tissue plays an important role in tooth preservation by structurally maintaining the connection between the tooth root and the bone. The mechanisms involved in the healing and regeneration of damaged PDL tissue, caused by bacterial infection, caries and trauma, have been explored. Accumulating evidence suggests that Activin A, a member of the transforming growth factor-β (TGF-β) superfamily and a dimer of inhibinβa, contributes to tissue healing through cell proliferation, migration, and differentiation of various target cells. In bone, Activin A has been shown to exert an inhibitory effect on osteoblast maturation and mineralization. However, there have been no reports examining the expression and function of Activin A in human PDL cells (HPDLCs). Thus, we aimed to investigate the biological effects of Activin A on HPDLCs. Activin A was observed to be localized in HPDLCs and rat PDL tissue. When PDL tissue was surgically damaged, Activin A and IL-1β expression increased and the two proteins were shown to be co-localized around the lesion. HPDLCs treated with IL-1β or TNF-α also up-regulated the expression of the gene encoding inhibinβa. Activin A promoted chemotaxis, migration and proliferation of HPDLCs, and caused an increase in fibroblastic differentiation of these cells while down-regulating their osteoblastic differentiation. These osteoblastic inhibitory effects of Activin A, however, were only noted during the early phase of HPDLC osteoblastic differentiation, with later exposures having no effect on differentiation. Collectively, our results suggest that Activin A could be used as a therapeutic agent for healing and regenerating PDL tissue in response to disease, trauma or surgical reconstruction.

Endocrine aspects of bone metastases

Skeletal lesions are a frequent complication of breast and prostate cancer and a hallmark of multiple myeloma. Endocrine and paracrine factors modulate various aspects of bone metastases, including tumour proliferation, skeletal susceptibility to tumour homing, the microenvironment needed to support tumour persistence, and the initiation of a vicious cycle between tumour and bone-resident cells that further promotes tumour growth. Endocrine changes, such as oestrogen or vitamin D deficiency, contribute to a fertile bone microenvironment that might promote bone metastases. Bone health could be impaired further by existing cancer treatments, especially sex hormone deprivation. In this Review, we discuss the effect of hormones and associated local factors on cross-talk between bone metabolism and tumour biology. We review the biology of osteolytic and osteosclerotic lesions, with a focus on endocrine aspects, and outline potential therapeutic targets. We also summarise endocrine aspects of the pathogenesis and clinical presentation of bone metastases and provide an update on existing and future treatments.

Circulating activin-A is elevated in postmenopausal women with low bone mass: the three-month effect of zoledronic acid treatment

Activin-A is expressed in bone and seems to regulate osteoclastogenesis. In this study, serum activin-A was increased in postmenopausal women with low bone mass and was positively correlated to age and negatively to lumbar spinal bone mineral density (BMD). Serum activin-A levels did not change 3 months after zoledronic acid infusion.

INTRODUCTION

The aims of the study were to evaluate prospectively the circulating activin-A levels in postmenopausal women with low bone mass and explore possible correlations with clinical and laboratory data, as well as the 3-month effect of zoledronic acid infusion.

METHODS

Postmenopausal women with low bone mass assigned to receive zoledronic acid infusion (Patients, n = 47) and age-matched, postmenopausal women with normal bone mass (Controls, n = 27) were recruited on an outpatient basis. Main outcome measurement was serum activin-A levels.

RESULTS

Serum activin-A was higher in patients at baseline compared to controls (p < 0.001) and activin-A in the serum of patients and controls was positively correlated with age (Spearman's coefficient of correlation [rs] = 0.325; p = 0.005) and negatively with lumbar spinal (LS) BMD (rs = -0.425; p < 0.001). In multiple linear regression analysis, only age (B = 8.93; 95 % CI = 4.39-13.46; p < 0.001) was associated with serum activin-A levels at baseline, independent from group (patients or controls), previous anti-osteoporotic treatment, LS BMD and follicle-stimulating hormone. Circulating activin-A levels were not affected 3 months after zoledronic acid infusion.

CONCLUSIONS

Serum activin-A is increased in postmenopausal women with low bone mass compared with postmenopausal women with normal bone mass and is positively correlated to age and negatively to LS BMD.

Activin A suppresses osteoblast mineralization capacity by altering extracellular matrix (ECM) composition and impairing matrix vesicle (MV) production

During bone formation, osteoblasts deposit an extracellular matrix (ECM) that is mineralized via a process involving production and secretion of highly specialized matrix vesicles (MVs). Activin A, a transforming growth factor-β (TGF-β) superfamily member, was previously shown to have inhibitory effects in human bone formation models through unclear mechanisms. We investigated these mechanisms elicited by activin A during in vitro osteogenic differentiation of human mesenchymal stem cells (hMSC). Activin A inhibition of ECM mineralization coincided with a strong decline in alkaline phosphatase (ALP(1)) activity in extracellular compartments, ECM and matrix vesicles. SILAC-based quantitative proteomics disclosed intricate protein composition alterations in the activin A ECM, including changed expression of collagen XII, osteonectin and several cytoskeleton-binding proteins. Moreover, in activin A osteoblasts matrix vesicle production was deficient containing very low expression of annexin proteins. ECM enhanced human mesenchymal stem cell osteogenic development and mineralization. This osteogenic enhancement was significantly decreased when human mesenchymal stem cells were cultured on ECM produced under activin A treatment. These findings demonstrate that activin A targets the ECM maturation phase of osteoblast differentiation resulting ultimately in the inhibition of mineralization. ECM proteins modulated by activin A are not only determinant for bone mineralization but also possess osteoinductive properties that are relevant for bone tissue regeneration.

Lenalidomide in combination with an activin A-neutralizing antibody: preclinical rationale for a novel anti-myeloma strategy

Given the prevalence of osteolytic bone disease in multiple myeloma (MM), novel therapies targeting bone microenvironment are essential. Previous studies have identified activin A to be of critical importance in MM-induced osteolysis. Lenalidomide is a known and approved treatment strategy for relapsed MM. Our findings demonstrate that lenalidomide acts directly on bone marrow stromal cells via an Akt-mediated increase in Jun N-terminal kinase-dependent signaling resulting in activin A secretion, with consequent inhibition of osteoblastogenesis. Here, we attempted to augment the antitumor benefits of lenalidomide while overcoming its effects on osteoblastogenesis by combining it with a neutralizing antibody to activin A. Increased activin A secretion induced by lenalidomide was abrogated by the addition of activin A-neutralizing antibody, which effectively restored osteoblast function and inhibited MM-induced osteolysis without negating the cytotoxic effects of lenalidomide on malignant cells. This provides the rationale for an ongoing clinical trial (NCT01562405) combining lenalidomide with an anti-activin A strategy.

Mechanisms involved in inhibition of chondrogenesis by activin-A

OBJECTIVES

Activin-A, a member of the TGF-β family, is known to be present in bone and cartilage. Although, involvement of the TGF-β family in chondrogenesis has been reported, the mechanism by which activin-A regulates chondrogenesis has not been fully elucidated. The aim of this study was to investigate the effects of activin-A on chondrocyte differentiation in vitro.

MATERIALS AND METHODS

Monolayer cultures of mouse chondrocyte ATDC cells were pretreated with a variety of inhibitors of major signaling pathways prior to addition of activin-A. The expressions of sox9, runx2, and osterix mRNA were detected using real-time PCR. To determine chondrocyte differentiation, sulfated glycosaminoglycans were stained with Alcian blue. To further elucidate the role of activin-A on chondrogenesis regulation, phosphorylation of Smad2/3, ERK, JNK, and Akt proteins was determined by western blotting.

RESULTS

Activin-A suppressed the transcription of sox9, runx2, and osterix mRNA, as well as sulfated glycosaminoglycans accumulation. Activin-A also inhibited constitutive phosphorylation of JNK and Akt proteins. Furthermore, inhibition of the JNK and PI3K-Akt pathways by chemical inhibitors suppressed chondrogenesis in ATDC5 cells.

CONCLUSIONS

These results indicate that activin-A may suppress chondrocyte differentiation in ATDC5 cells via down-regulation of JNK and Akt phosphorylation.

Biphasic effects of transforming growth factor β on bone morphogenetic protein-induced osteoblast differentiation

Bone morphogenetic proteins (BMPs) exert an important role in skeletal development, adult bone homeostasis, and fracture healing and have demonstrated clinical utility for bone regeneration. However, BMPs fall short as regenerative agents because high doses need to be used to obtain therapeutic effects. Determining the molecular mechanisms controlling BMP-induced bone formation may lead to the development of more effective BMP-based therapies. To identify kinases mediating BMP-induced osteoblast differentiation, we performed an siRNA screen to find kinases modulating BMP-6-induced alkaline phosphatase (ALP) activity. Surprisingly, although transforming growth factor β (TGF-β) generally is considered to antagonize BMP-induced osteoblast differentiation, C2C12  cells transfected with siRNAs targeting TGF-β receptors displayed reduced BMP-6-induced ALP activity. Furthermore, pharmacologic inhibitors blocking the TGF-β type I receptor impaired BMP-induced ALP activity in KS483 and C2C12 cells and mineralization of KS483 cells. Consistently, costimulation with BMPs and TGF-β further increased expression of osteoblast-specific genes, ALP activity, and mineralization of KS483 cells and primary mesenchymal stem cells compared with BMPs alone. The stimulatory and inhibitory effects of TGF-β were found to depend on timing and duration of the costimulation. TGF-β inhibited BMP-induced activation of a BMP-Smad-dependent luciferase reporter, suggesting that the stimulatory effect of TGF-β is not due to increased BMP-Smad activity. TGF-β also inhibited the BMP-induced expression of the BMP antagonist noggin and prolonged BMP activity. In conclusion, TGF-β, besides acting as an inhibitor, also can, by dampening the noggin-mediated negative-feedback loop, enhance BMP-induced osteoblast differentiation, which might be beneficial in fracture healing.

Inhibiting activin-A signaling stimulates bone formation and prevents cancer-induced bone destruction in vivo

Cancers that grow in bone, such as myeloma and breast cancer metastases, cause devastating osteolytic bone destruction. These cancers hijack bone remodeling by stimulating osteoclastic bone resorption and suppressing bone formation. Currently, treatment is targeted primarily at blocking bone resorption, but this approach has achieved only limited success. Stimulating osteoblastic bone formation to promote repair is a novel alternative approach. We show that a soluble activin receptor type IIA fusion protein (ActRIIA.muFc) stimulates osteoblastogenesis (p < .01), promotes bone formation (p < .01) and increases bone mass in vivo (p < .001). We show that the development of osteolytic bone lesions in mice bearing murine myeloma cells is caused by both increased resorption (p < .05) and suppression of bone formation (p < .01). ActRIIA.muFc treatment stimulates osteoblastogenesis (p < .01), prevents myeloma-induced suppression of bone formation (p < .05), blocks the development of osteolytic bone lesions (p < .05), and increases survival (p < .05). We also show, in a murine model of breast cancer bone metastasis, that ActRIIA.muFc again prevents bone destruction (p < .001) and inhibits bone metastases (p < .05). These findings show that stimulating osteoblastic bone formation with ActRIIA.muFc blocks the formation of osteolytic bone lesions and bone metastases in models of myeloma and breast cancer and paves the way for new approaches to treating this debilitating aspect of cancer.

Follistatin as a potent regulator of bone metabolism

Follistatin is a monomeric glycoprotein, distributed in a wide range of tissues. Recent work has demonstrated that this protein is a pluripotential molecule that has no structural similarity but is functionally associated with members of the transforming growth factor (TGF)-β superfamily, which indicates its wide range of action. Members of the TGF-β superfamily, especially activins and bone morphogenetic proteins are involved in bone metabolism. They play an important role in bone physiology, influencing bone growth, turnover, bone formation and cartilage induction. As follistatin is considered to be the antagonist of the TGF-β superfamily members, it plays an important role in bone metabolism and development.

Activin A expression regulates multipotency of mesenchymal progenitor cells

Introduction

Bone marrow (BM) stroma currently represents the most common and investigated source of mesenchymal progenitor cells (MPCs); however, comparable adult progenitor or stem cells have also been isolated from a wide variety of tissues. This study aims to assess the functional similarities of MPCs from different tissues and to identify specific factor(s) related to their multipotency.

Methods

For this purpose, we directly compared MPCs isolated from different adult tissues, including bone marrow, tonsil, muscle, and dental pulp. We first examined and compared proliferation rates, immunomodulatory properties, and multidifferentiation potential of these MPCs in vitro. Next, we specifically evaluated activin A expression profile and activin A:follistatin ratio in MPCs from the four sources.

Results

The multidifferentiation potential of the MPCs is correlated with activin A level and/or the activin A:follistatin ratio. Interestingly, by siRNA-mediated activin A knockdown, activin A was shown to be required for the chondrogenic and osteogenic differentiation of MPCs. These findings strongly suggest that activin A has a pivotal differentiation-related role in the early stages of chondrogenesis and osteogenesis while inhibiting adipogenesis of MPCs.

Conclusions

This comparative analysis of MPCs from different tissue sources also identifies bone marrow-derived MPCs as the most potent MPCs in terms of multilineage differentiation and immunosuppression, two key requirements in cell-based regenerative medicine. In addition, this study implicates the significance of activin A as a functional marker of MPC identity.

A soluble activin receptor Type IIA fusion protein (ACE-011) increases bone mass via a dual anabolic-antiresorptive effect in Cynomolgus monkeys

Activin A belongs to the TGF-beta superfamily and plays an important role in bone metabolism. It was reported that a soluble form of extracellular domain of the activin receptor type IIA (ActRIIA) fused to the Fc domain of murine IgG, an activin antagonist, has an anabolic effect on bone in intact and ovariectomized mice. The present study was designed to examine the skeletal effect of human ActRIIA-IgG1-Fc (ACE-011) in non-human primates. Young adult female Cynomolgus monkeys were given a biweekly subcutaneous injection of either 10mg/kg ACE-011 or vehicle (VEH) for 3months. Treatment effects were evaluated by histomorphometric analysis of the distal femur, femoral midshaft, femoral neck and 12th thoracic vertebrae, by muCT analysis of femoral neck and by biomarkers of bone turnover. Compared to VEH, at the distal femur ACE-011-treated monkeys had significantly increased cancellous bone volume (+93%), bone formation rate per bone surface (+166%) and osteoblast surface (+196%) indicating an anabolic action. Monkeys treated with ACE-011 also had decreased osteoclast surface and number. No differences were observed in parameters of cortical bone at the midshaft of the femur. Similar to distal femur, ACE-011-treated monkeys had significantly greater cancellous bone volume, bone formation rate and osteoblast surface at the femoral neck relative to VEH. A significant increase in bone formation rate and osteoblast surface with a decrease in osteoclast surface was observed in thoracic vertebrae. muCT analysis of femoral neck indicated more plate-like structure in ACE-011-treated monkeys. Monkeys treated with ACE-011 had no effect on serum bone-specific alkaline phosphatase and CTX at the end of the study. These observations demonstrate that ACE-011 is a dual anabolic-antiresorptive compound, improving cancellous bone volume by promoting bone formation and inhibiting bone resorption in non-human primates. Thus, soluble ActRIIA fusion protein may be useful in the prevention and/or treatment of osteoporosis and other diseases involving accelerated bone loss.

Activin A promotes multiple myeloma-induced osteolysis and is a promising target for myeloma bone disease

Understanding the pathogenesis of cancer-related bone disease is crucial to the discovery of new therapies. Here we identify activin A, a TGF-beta family member, as a therapeutically amenable target exploited by multiple myeloma (MM) to alter its microenvironmental niche favoring osteolysis. Increased bone marrow plasma activin A levels were found in MM patients with osteolytic disease. MM cell engagement of marrow stromal cells enhanced activin A secretion via adhesion-mediated JNK activation. Activin A, in turn, inhibited osteoblast differentiation via SMAD2-dependent distal-less homeobox-5 down-regulation. Targeting activin A by a soluble decoy receptor reversed osteoblast inhibition, ameliorated MM bone disease, and inhibited tumor growth in an in vivo humanized MM model, setting the stage for testing in human clinical trials.

Treatment with a soluble receptor for activin improves bone mass and structure in the axial and appendicular skeleton of female cynomolgus macaques (Macaca fascicularis)

A recent study suggests that activin inhibits bone matrix mineralization, whereas treatment of mice with a soluble form of the activin type IIA receptor markedly increases bone mass and strength. To further extend these observations, we determined the skeletal effects of inhibiting activin signaling through the ActRIIA receptor in a large animal model with a hormonal profile and bone metabolism similar to humans. Ten female cynomolgus monkeys (Macaca fascicularis) were divided into two weight-matched groups and treated biweekly, for 3 months, with either a subcutaneous injection 10 mg/kg of a soluble form of the ActRIIA receptor fused with the Fc portion of human IgG(1) (ACE-011) or vehicle (VEH). Bone mineral density (BMD), micro-architecture, compressive mechanical properties, and ash fraction were assessed at the end of the treatment period. BMD was significantly higher in ACE-011 treated individuals compared to VEH: +13% (p=0.003) in the 5th lumbar vertebral body and +15% (p=0.05) in the distal femur. In addition, trabecular volumetric bone density at the distal femur was 72% (p=0.0004) higher than the VEH-treated group. Monkeys treated with ACE-011 also had a significantly higher L5 vertebral body trabecular bone volume (p=0.002) and compressive mechanical properties. Ash fraction of L4 trabecular bone cores did not differ between groups. These results demonstrate that treatment with a soluble form of ActRIIA (ACE-011) enhances bone mass and bone strength in cynomolgus monkeys, and provide strong rationale for exploring the use of ACE-011 to prevent and/or treat skeletal fragility.

Regulation of osteoblastogenesis and osteoclastogenesis by the other reproductive hormones, Activin and Inhibin

There is both cellular and physiological evidence demonstrating that both Activins and Inhibins regulate osteoblastogenesis and osteoclastogenesis, and regulate bone mass in vivo. Although Activins and Inhibins were initially isolated from the gonad, Activins are also produced and stored in bone, whereas Inhibins exert their regulation on bone cell differentiation and metabolism via endocrine effects. The accumulating data provide evidence that reproductive hormones, distinct from classical sex steroids, are important regulators of bone mass and bone strength. Given the well described dominant antagonism of Inhibin over Activin, as well as over BMPs and TGFbeta, the gonadally derived Inhibins are important regulators of locally produced osteotrophic factors. Thus, the cycling Inhibins in females and diurnal changes in Inhibin B in males elicit temporal shifts in Inhibin levels (tone) that de-repress the pituitary. This fundamental action has the potential to de-repress locally stimulated changes in osteoblastogenesis and osteoclastogenesis, thereby altering bone metabolism.

Biological functions of miR-29b contribute to positive regulation of osteoblast differentiation

Bone tissue arises from mesenchymal cells induced into the osteoblast lineage by essential transcription factors and signaling cascades. MicroRNAs regulate biological processes by binding to mRNA 3'-untranslated region (UTR) sequences to attenuate protein synthesis. Here we performed microRNA profiling and identified miRs that are up-regulated through stages of osteoblast differentiation. Among these are the miR-29, miR-let-7, and miR-26 families that target many collagens and extracellular matrix proteins. We find that miR-29b supports osteoblast differentiation through several mechanisms. miR-29b decreased and anti-miR-29b increased activity of COL1A1, COL5A3, and COL4A2 3'-UTR sequences in reporter assays, as well as endogenous gene expression. These results support a mechanism for regulating collagen protein accumulation during the mineralization stage when miR-29b reaches peak levels. We propose that this mechanism prevents fibrosis and facilitates mineral deposition. Our studies further demonstrate that miR-29b promotes osteogenesis by directly down-regulating known inhibitors of osteoblast differentiation, HDAC4, TGFbeta3, ACVR2A, CTNNBIP1, and DUSP2 proteins through binding to target 3'-UTR sequences in their mRNAs. Thus, miR-29b is a key regulator of development of the osteoblast phenotype by targeting anti-osteogenic factors and modulating bone extracellular matrix proteins.

Constitutively activated ALK2 and increased SMAD1/5 cooperatively induce bone morphogenetic protein signaling in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder characterized by congenital malformation of the great toes and by progressive heterotopic bone formation in muscle tissue. Recently, a mutation involving a single amino acid substitution in a bone morphogenetic protein (BMP) type I receptor, ALK2, was identified in patients with FOP. We report here that the identical mutation, R206H, was observed in 19 Japanese patients with sporadic FOP. This mutant receptor, ALK2(R206H), activates BMP signaling without ligand binding. Moreover, expression of Smad1 and Smad5 was up-regulated in response to muscular injury. ALK2(R206H) with Smad1 or Smad5 induced osteoblastic differentiation that could be inhibited by Smad7 or dorsomorphin. Taken together, these findings suggest that the heterotopic bone formation in FOP may be induced by a constitutively activated BMP receptor signaling through Smad1 or Smad5. Gene transfer of Smad7 or inhibition of type I receptors with dorsomorphin may represent strategies for blocking the activity induced by ALK2(R206H) in FOP.

Matrix and TGF-β-related gene expression during human dental pulp stem cell (DPSC) mineralization

We have recently reported the induction of dental pulp stem cells (DPSCs) into dentin-secreting odontoblast-like cells after stimulation by isolated dentin matrix components, thus mimicking the nature of tissue regeneration seen after tooth disease and injury. After confluency, the cells were further cultured for 21 d in the 10% fetal bovine serum (FBS) Dulbecco's modified Eagle's medium (DMEM) (control), and in this medium, with the addition of dentin extract (DE) and the mineralization supplement (MS) of ascorbic acid and beta-glycerophosphate (treatment). To identify genes associated with this process, specimens were analyzed with a HG-U133A human gene chip and Arrayassist software. A total of 425 genes, among them 21 matrix and eight TGF-beta-related genes, were either up- or downregulated in the experimental group in which the cells showed odontoblast-like differentiation and mineralization. Expression of selected genes was further confirmed by real-time polymerase chain reaction (PCR) analysis. Of the extracellular matrix (ECM)-related genes, two types of collagen genes were upregulated and seven others downregulated. Other ECM-related genes, for example fibulin-1, tenascin C, and particularly thrombospondin 1, were upregulated, and fibulin-2 was downregulated. Most noticeably, the matrix metalloproteinase 1 was induced by the treatment. In the TGF-beta superfamily, upregulation of the type II receptor, endoglin, and growth/differentiation factor 5 was coordinated with the downregulation of activin A, TGF-beta2, and TGF-beta1 itself. This study identifies the matrix and TGF-beta-related gene profiles during the DPSC cell mineralization in which several genes are reported for the first time to be associated with this process, thus greatly expanding our molecular knowledge of the induced disease repair process.

The activin A-follistatin system: potent regulator of human extracellular matrix mineralization

Bone quality is an important determinant of osteoporosis, and proper osteoblast differentiation plays an important role in the control and maintenance of bone quality. We investigated the impact of activin signaling on human osteoblast differentiation, extracellular matrix formation, and mineralization. Activins belong to the transforming growth factor-beta superfamily and activin A treatment strongly inhibited mineralization in osteoblast cultures, whereas the activin antagonist follistatin increased mineralization. Osteoblasts produced activin A and follistatin in a differentiation-dependent manner, leading to autocrine regulation of extracellular matrix formation and mineralization. In addition, mineralization in a vascular smooth muscle cell-based model for pathological calcification was inhibited. Comparative activin A and follistatin gene expression profiling showed that activin signaling changes the expression of a specific range of extracellular matrix proteins prior to the onset of mineralization, leading to a matrix composition with reduced or no mineralizing capacity. These findings demonstrate the regulation of osteoblast differentiation and matrix mineralization by the activin A-follistatin system, providing the possibility to control bone quality as well as pathological calcifications such as atherosclerosis by using activin A, follistatin, or analogs thereof.

Follistatin restricts bone morphogenetic protein (BMP)-2 action on the differentiation of osteoblasts in fetal rat mandibular cells

We tested whether FS secretion might modulate BMP-2 actions by measuring FS levels and counting bone numbers of rat mandibular cells. In the presence of Dex, BMP-2 stimulated FS secretion at the early phase and augmented bone nodule by neutralizing with FS antibody. We concluded that BMP-2 facilitates FS secretion, and the FS restricts BMP-2 action on osteoblastogenesis.

INTRODUCTION

Bone morphogenetic proteins (BMPs) promote the differentiation of osteoprogenitor cells into osteoblasts. Activin A is involved in the regulation of bone formation. Follistatin (FS) antagonizes the bioactivities of BMP and activin A extracellularly.

MATERIALS AND METHODS

In this study, we tested whether the induction of FS secretion might modulate the effects of BMP-2 on osteoblast development, using the bone nodule-forming cultures of fetal rat mandibular cells.

RESULTS AND CONCLUSIONS

In the presence of dexamethasone (Dex), BMP-2 stimulated the secretion of FS at the early phase (days 3-9) of the culture. Dex alone had no effect, and BMP-2 alone was less effective than the combination of the two. BMP-4 and -6 had little effect on FS secretion. Activin A inhibited the early upregulation of FS secretion when added with BMP-2 and Dex. In the presence of Dex, BMP-2 increased bone nodule numbers when added to early cultures. The addition of anti-FS antibody to cultures with BMP-2 and Dex augmented bone nodule formation. These results show that BMP-2 facilitates the secretion of FS in the presence of Dex, and the increased FS secretion restricts the action of BMP-2 on osteoblast differentiation.

Microarray analysis of bone morphogenetic protein, transforming growth factor beta, and activin early response genes during osteoblastic cell differentiation

Bone morphogenetic protein (BMP) 2, a member of the transforming growth factor (TGF) beta family, is a potent regulator of osteoblast differentiation. In addition, both TGF-beta and activin A can either induce bone formation or inhibit bone formation depending on cell type and differentiation status. Although much is known about the receptors and intracellular second messengers involved in the action of TGF-beta family members, little is known about how selectivity in the biological response of individual family members is controlled. In this study, we have investigated selective gene induction by BMP-2, TGF-beta1 and activin A in relation to their ability to control differentiation of mouse mesenchymal precursor cells C2C12 into osteoblastic cells. TGF-beta1 can inhibit BMP-2-induced differentiation of these cells, whereas activin A was found to be without morphogenetic effect. Using a gene expression microarray approach covering 8636 sequences, we have identified a total of 57 established genes and expressed sequence tags (ESTs) that were either up-regulated or down-regulated 2 h after treatment with at least one of these three stimuli. With respect to the established genes, 15 new target genes for TGF-beta family members thus were identified. Furthermore, a set of transcripts was identified, which was oppositely regulated by TGF-beta1 and BMP-2. Based on the inverse biological effects of TGF-beta1 and BMP-2 on C2C12 cells, these genes are important candidates for controlling the process of growth factor-induced osteoblast differentiation.

Identification and functional characterization of distinct critically important bone morphogenetic protein-specific response elements in the Id1 promoter

Transforming growth factor-beta (TGF-beta) family members, which include bone morphogenetic proteins (BMPs) and TGF-betas, elicit their cellular effects by activating specific Smad proteins, which control the transcription of target genes. BMPs and TGF-betas have overlapping as well as specific effects on mesenchymal cell differentiation for which the mechanisms are incompletely understood. Here we report that Id1, a dominant negative inhibitor of basic helix-loop-helix proteins, is a direct target gene for BMP. BMP, but not TGF-beta, strongly activates the Id1 promoter in an Smad-dependent manner. We identified two BMP-responsive regions in the mouse Id1 promoter, which contain three distinct sequence elements; one region contains two Smad binding elements (SBEs), and the other region contains a GGCGCC palindromic sequence flanked by two CAGC and two CGCC motifs. Whereas SBEs and GGCGCC sequence are critically important, the CAGC and CGCC motifs are needed for efficient BMP-induced Id1 promoter activation. Smads are part of nuclear transcription factor complexes that specifically bind to SBEs and GGCGCC sequence in response to BMP but not TGF-beta. Multimerization of the all three distinct sequence motifs is needed to generate a highly sensitive and BMP/Smad-dependent specific enhancer. Our results provide important new insights into how the BMP/Smad pathway can specifically activate target genes.

Inhibin suppresses and activin stimulates osteoblastogenesis and osteoclastogenesis in murine bone marrow cultures

Using primary murine bone marrow cell cultures, we demonstrate that inhibin suppresses osteoblastogenesis and osteoclastogenesis. In contrast, activin supports osteoblast formation (by alkaline phosphatase-positive and mineralized colony formation); and activin also stimulates osteoclast formation (as measured by staining tartrate-resistant acid phosphatase-positive multinucleated cells). Inhibin, the activin antagonist follistatin, and the bone morphogenetic protein antagonist noggin can all suppress endogenous activin accumulation in bone marrow cultures. Associated with this decrease in activin is the loss of mineralized osteoblastic colony formation (colony forming unit-osteoblast; CFU-OB). However, exogenous activin administration, even in the presence of noggin, permits both alkaline phosphatase-positive and CFU-OB colony formation in vitro. In contrast, the stimulatory effects of locally produced activin on osteoblast and osteoclast development are not likely to be dominant over the suppressive effects of gonadally derived inhibin. The suppressive effect of inhibin is maintained in the presence of either activin or bone morphogenetic protein, suggesting the presence of a distinct inhibin-specific receptor. Taken together, the direct regulation of osteoblastogenesis and osteoclastogenesis by inhibin and activin in vitro suggest that changes in the inhibin/activin ratio detected by bone marrow cells, during the perimenopausal transition, contribute to altered cell differentiation and may be associated with the increased bone resorption observed at this time.

Immature osteoblastic cells express the pro-alpha2(XI) collagen gene during bone formation in vitro and in vivo

Type XI collagen is predominantly found in cartilage. However, expression of the pro-alpha2(XI) collagen gene (COL11A2) has recently been detected in various non-cartilaginous tissues. We identified the differentiation stage at which COL11A2 was expressed in cultured fetal rat calvarial (FRC) cells and in rat femoral fracture calluses in order to investigate the involvement of COL11A2 during bone formation in vitro and in vivo. We also studied the alternative splicing of exons 6-8 in FRC cells and fracture calluses. In FRC cells, mineralized nodules stained with von Kossa stain were observed from day 9 after confluence. COL11A2 was highly expressed on days 0 and 5, but the expression levels were rapidly decreased on day 9 by Northern blot analysis. During rat femoral fracture repair, intramembranous ossification proceeded and newly formed woven bone was observed on the cortex on day 7 after fracture. In situ hybridization showed that COL11A2 signals were detected in osteoblastic cells in the newly formed woven bone. According to the maturation and remodeling of the woven bone into the trabecular bone, the distribution of the signal for COL11A2 mRNA was limited to the superficial osteoblastic cells of the newly formed trabecular bone. These results demonstrated that COL11A2 was expressed in relatively immature osteoblastic cells during bone formation in vitro and in vivo. RT-PCR showed that the shortest band corresponding to mRNA lacking exons 6-8 was clearly detected when using RNA from soft calluses. In contrast, the largest band corresponding to mRNA with exons 6-8 was predominant when using RNA from FRC cells or from hard calluses on days 7 and 14. These results indicate that the splicing pattern of exons 6-8 in osteoblastic cells is different from the pattern in chondrocytes.